Arcane Trickster as an actual Rogue Archetype would be swell.
I know there's a LOT of other archetypes that would fit for Rogues, but this one came to mind because it's one of the first prestige classes ever released.
For Wizards, I know it probably wouldn't fit today's run-n-gun gameplay, but I'd like a Geometer specializing in trap spells like fire trap and explosive runes.
For Fighters, because we're not that likely to get the actual class, I'd like to see Samurai introduced. Now that Flame Blades and the likes got fixed to get better as you level, giving Fighters slightly less feats for the power of an Ancestral Daisho? Cool.
Shame is it'd be REALLY hard to make it "feel" new beyond this.
Barbarians? The most iconic classes (to me) always were Frenzied Berserker, which we already have, and Bear Warriors, which Druids kinda took over.
I'd like to see DDO's take on Lion or Wolf Totem Barbarians. Being constantly on the move giving them bonuses they lose if they stop running would make their gameplay unique at least.